Thanks for the comments they lead me to the following.. very crude, but am I on the right track for a basic approximation?
1. Specific heat of aluminium = 913J/Kg.K
2. Energy to be dissipated = 1000J
3. Heat energy = mass x specific heat capacity x temperature change
for a 50degC rise in temp of the aluminium (I'll assume a solid block) the following amount of material is required..
mass = 1000J/(913*50) = 22 grams (not much!)
Area under the power curve is energy so the approximate temp rise each second for a 22g block of aluminium.. (???)
in the first second approx. 200J of heat energy is transferred to the block:
200/(0.022*913) = 10degC (Ta + 10 = Tt = 30degC)
the second second approx. 150J of heat energy is transferred to the block:
150/(0.022*913) = 7.5degC (Tt + 7.5 = 37.5degC)
etc...
I should be able to work backwards through the various thermal resistances to then estimate the junction temp... I'll prototype it and see how well this idea approximates the real thing.. Although I fear my simplistic idea assumes the heat energy is distributed uniformly, and as FvM points out this is not the case.. There will be a thermal gradient throughout the aluminium... which will determine the rate at which the heat energy is distributed throughout, which is the more complex part of the equation that I've conveniently left out and hope to determine with a few actual measurements